1. Field of the Invention
This invention relates to teletext decoders for receiving, storing and processing teletext information which is transmitted as digitally coded data.
2. Related Art
A teletext decoder which is suitable for producing character display data for a teletext display page includes an acquisition circuit for acquiring digital codes representing teletext display information for the page, a page memory for storing these acquired digital codes, and a character generator for producing character display data in accordance with the stored digital codes.
Conventionally, the character generator includes a character memory in which is stored character information representing the character shapes which are available for display. The character memory is selectively addressed using the digital codes stored in the page memory to read out the character display data. The character information for each character shape is stored in at least one individual character memory location which is addressable by a respective digital code. Normally, only a portion of the character information for a character shape is read out at any one time as the character display data, which portion is displayed in a current scanning line of the display.
Current transmissions of teletext information are in broadcast television signals. These teletext transmissions in the United Kingdom are essentially only in the English language, and a character generator which is used for these transmissions includes a character memory containing the character information for an English language character set. Where teletext transmissions are provided in a country whose national language uses a different character set, a character generator which is used for these latter transmissions can include a character memory containing the character information for a character set for the language concerned.
The character requirements of the major European languages differ only in a few (.about.11) national use character options. Therefore, these requirements can be met by a single composite character memory containing the character information for a common character set and a plurality of national option character sub-sets each of which sub-sets forms with the common character set a complete language character set for a respective language.
In current teletext transmissions, control information which forms part of the teletext information for a page identifies which language character set should be used to display the page. Where a teletext decoder has a character generator with a character memory containing, as aforesaid, a common character set and a plurality of national option character sub-sets, this control information is used in the teletext decoder to select a particular sub-set from those which are available and thereby in effect select a single language character set.
As is known from Applicants GB patent specification No. 2 149 627, a small group of the total number of the possible digital codes representing the teletext information may be allocated in common for identifying different character shapes in several national option character sub-sets, the character information for each of which is stored in a respective block of character memory locations. The control information is used to determine which block is to be addressed in respect of acquired digital codes belonging to this small group. If these acquired digital codes as stored in the page memory are not codes which can themselves address memory locations in the selected block, then code conversion is carried out when these digital codes are read out from the page memory to convert them into the appropriate codes for this addressing. To achieve this, each of the digital codes in the small group has to be identified separately so as to be code converted, or not, in accordance with the control information.
The selection of a particular one of several national option character sub-sets by means of the control information has the limitation that it is not possible to mix languages on a single page. Another more serious limitation occurs in connection with a facility proposed for teletext decoders which are micro-processor controlled, whereby locally-generated status messages can be displayed under the control of the microprocessor on an additional display row either alone or with a displayed page which contains received broadcast display rows. Such a status message should be in a given language which should not change even though the language of received broadcast pages may vary. Thus, this status message facility can require a mixed language page display which cannot be achieved using said control information.
In order to overcome these limitations, Applicants GB patent specification No. 2149627 mentioned previously discloses a teletext decoder having a character memory in which is stored character information in the form of a common character set and a plurality of national option character sub-sets. This teletext decoder functions according to a first addressing mode in which any one of said sub-sets can be chosen by control information contained in received teletext information to form with the common character set a complete language character set from which characters can be selected to provide for page display in only one language, and a second addressing mode in which all of the sub-sets can be chosen to form with the common character set a corresponding plurality of complete language character sets from all of which characters can be selected to provide for page display in more than one language.
In a specific implementation of this teletext decoder, the addressing of characters in both addressing modes is effected using different 7-bit codes. A major portion of the total possible number of the 7-bit codes are used in both modes to select respective characters in the common language character set. In the second mode most of the remaining 7-bit codes are used to select respective characters in all of the national option character sub-sets However, in the first mode only a few of these remaining 7-bit codes, sufficient for selecting the characters of only one national option sub-set, are available for character selection because the others are required instead for control purposes. As a consequence, the control information contained in received teletext information is used to choose by means of code conversion (or not) which single national option sub-set can have its characters selected in respect of said few remaining 7-bit codes. An eighth bit is associated with each 7-bit code. When this eighth bit is set to logic 0, the first addressing mode obtains, and when it is set to logic 1, the second addressing mode obtains. This eighth bit is therefore used merely as a "toggle" to determine whether most or only a few of said remaining 7-bit codes are to be used for selecting the characters of the national option sub-sets. The number of characters that can be provided in each national option sub-set is limited by the number of 7-bit codes that remain for selecting these characters after the allocation of respective 7-bit codes for selecting the characters of the common language set.
The implementation of the teletext decoder described in Applicants aforementioned GB patent specification No. 2149627 for processing the 7-bit codes which represent different characters is based on the premise that 2.sup.7 =128 different codes are available for character selection, this being the number of possible different 7-bit codes that can be received by the teletext decoder from a transmission source. In conformity with this premise, and allotting 32 of these codes for selecting control characters, the character memory has 96 memory locations, containing different character shapes, which can be addressed by a respective one of the 96 remaining codes. In the embodiment described, the character memory also has a further 32 memory locations containing further character shapes, mainly for national option characters. In order to address these further memory locations, the eighth bit associated with each of the thirty-two 7-bit codes concerned is set to logic 1, as aforesaid, in order to distinguish from when these 7-bit codes are used to select control characters. However, there is no need to distinguish, the 96 remaining 7-bit codes in any way because they are always used to address the same memory locations. Therefore, in respect of these remaining 7-bit codes, the eighth bit is a "don't care" bit in the sense that the same 96 memory locations will be addressed regardless of whether this bit is a logic 0 or a logic 1.